Analytic modelling of Rosetta Langmuir probe measurements based on SPIS simulations

Abstract: The Rosetta spacecraft is on route to rendezvous with the comet 67P/ Churyomov-Gerasimenko. One of the instruments onboard is the Langmuir probe instrument (LAP) developed by the Swedish Institute of Space Physics. The LAP includes two spherical probes used to measure a number of properties of the surrounding plasma. One measured property is the plasma density for which the spacecraft potential is a good proxy. By the determining the potential between spacecraft and Langmuir probe, the spacecraft potential can be measured. But such measurements are sometimes disturbed by the potential from the spacecraft itself, the influence from photoemitted electrons and the solar wind wake behind Rosetta. In order to correct for the errors caused by spacecraft-plasma interaction in the solar wind a model depending on the spacecraft (and Langmuir probe) orientation is developed. The data is obtained from three-dimensional simulations of Rosetta, with varying plasma parameters, using the software SPIS (Spacecraft Plasma Interaction System). Least squares fitting with a set of basis functions then provide the input for a parametric modelling. The developed model makes it possible to estimate the influence of each of the disturbing effects. The developed model gives good fits to data obtained in SPIS simulations. The two angular dependent factors, modelling perturbation on potential measurements caused by photoelectrons and wake effects, show errors below the 100 mV level in all cases. The model describing the influence from spacecraft potential is slightly less accurate, with errors at or below the 400 mV level in all relevant cases.